1. Field of the Invention
The present invention relates to a safety monitoring device for a robot that grips and carries a workpiece. Furthermore, the invention relates to a safety monitoring device for monitoring a robot provided with a force detection unit for detecting a force or torque acting on the robot.
2. Description of the Related Art
A force detection unit attached to a robot detects a force or a torque (hereinafter referred to as “external force” as appropriate) that the robot receives from an external environment to stop the robot or allow the robot to take an avoidance action on the basis of the detected external force. This can reduce a force that the robot applies to the external environment.
When such a robot carries a workpiece, a detection value of the force detection unit changes depending on whether or not the robot grips the workpiece. In other words, the detection value can change by an amount corresponding to a mass of the workpiece. Accordingly, in order to detect the external force accurately, it is necessary to switch the mass of the workpiece, a position of the center of gravity of the workpiece, and an inertia matrix of the workpiece that are used for calculation of the external force (hereinafter referred to as workpiece parameters), in accordance with the workpiece.
In addition, even when the robot grips a workpiece, the force detection unit may not be able to detect an entire mass of the workpiece. One example is a situation where the robot lifts a workpiece placed on a table. Since the workpiece is supported by the table, the force detection unit does not detect the mass of the workpiece at all or detects only a part of the mass thereof when the robot merely grips the workpiece. Then, when the robot lifts the workpiece, the workpiece departs from the table, thereby allowing the force detection unit to detect the entire mass of the workpiece. Accordingly, after the robot grips the workpiece and before the workpiece departs from the table, the force detection unit cannot accurately detect an external force.
The same applies to when the robot places the workpiece on the table. Specifically, in the situation where the robot places the workpiece, after the workpiece comes in contact with the table and before the robot releases the workpiece, the force detection unit cannot accurately detect an external force. Accordingly, regarding the operation of the robot that lifts the workpiece and the operation thereof that places the workpiece, it is difficult to monitor safety of the robot even with the monitoring of an external force.
Japanese Patent Publication No. 5088156 discloses a robot safety monitoring device for monitoring safety at a time of replacement of tools. In this patent literature, when current tool information as control information in a program does not match tool information of an actually attached tool, safety at the time of replacement thereof is monitored by monitoring whether or not the position of a robot is within a predetermined region.
In addition, Japanese Patent Publication No. 5154712 discloses a technique for maintaining a resting state of a robot when the robot grips or releases a workpiece. In this technique, when the robot grips the workpiece, switching between workpiece parameters is performed to prevent the robot from starting to move.
In this connection, a description will be given of monitoring regarding gripping of a workpiece in the same manner as in Japanese Patent Publication No. 5088156. Specifically, when a workpiece parameter currently used for external force calculation does not match a workpiece parameter of an actually gripped workpiece, it is monitored whether or not the position of the robot is within a predetermined region. However, as described above, in the operation of the robot that lifts the workpiece and the operation thereof that places the workpiece, external force may not be accurately detectable even though the workpiece parameter used for external force calculation matches the workpiece parameter of the actually gripped workpiece. Thus, employment of the technique of Japanese Patent Publication No. 5088156 has a problem with safety.
In addition, Japanese Patent Publication No. 5154712 has a problem in that no safety measures are taken when the robot performs the operation of lifting a gripped workpiece.
The present invention has been accomplished in view of such circumstances, and it is an object of the invention to provide a robot safety monitoring device that allows a robot to safely perform the operation of lifting a workpiece and the operation of placing the workpiece.